1. Field of the Invention
The present invention relates to a method of measuring the absorption or reflection of electromagnetic radiation by a material sample for a given wavelength of electromagnetic radiation with a beam source (1) in which radiation said material sample (3) is located and a beam detector (4) for detecting the radiation transmitted or reflected by said material sample, whereby the wavelength of the beam incident on said material sample is modulated by a first frequency (f.sub.1), whereby the modulated wavelength of said incident beam constitutes a first parameter, and a frequency-selective measurement is carried out phase-sensitive by at least one lock-in amplifier connected to said beam detector (4), it further relating to an apparatus for implementation thereof.
2. Description of the Prior Art
In optical spectroscopy there is often the problem of defining minimally small signals or signal changes of such optical parameters as absorption or reflection of electromagnetic radiation by a material sample. Absorption spectroscopy is one sensitive method of qualitative and quantitative analysis of trace elements in a material sample. The detection limit in absorption spectroscopy is given by the minimally detectable absorption determined by the signal-to-noise ratio. By employing modulation techniques (periodic change in characteristic parameters, such as e.g. amplitude, wavelength, etc and correlated measurement of the signal for modulation) it is possible to suppress or diminish noise signals as a function of the frequency (flicker noise). The resulting improvement in the signal-to-noise ratio produces a reduction in the detection limit.
One commonly employed technique is the periodic modulation of the wavelength of electromagnetic radiation which may be achieved, when laser diodes are used as the light sources, by periodically changing the laser diode current (see e.g. J. A. Silver, Applied Optics 31, 707 (1992) and the literature referenced therein). A laser beam modulated in such a way is passed through a material sample, e.g. a gas or plasma and is sensed by a beam detector, e.g. a photodiode in which a photocurrent is generated. The beam detector is coupled to a lock-in amplifier adjusted to the modulation frequency of the laser beam or the multiple thereof. By modulating the wavelength of the radiation of the laser diode in the kHz range it is possible to measure absorptions down to approx. 10.sup.-5 (for a detection bandwidth of 1 Hz).
The stated detection limit of 10.sup.-5 is determined by non-specific background signals caused by interference and etalon effects as well as by the amplitude modulation of the laser diode radiation, modulation of the laser diode current resulting namely not only in modulation of the wavelength but also modulation of the amplitude. The fluctations in the stated background signals determine the signal-to-noise ratio at the detection limit.